Fuel delivery systems for gas turbine engines in aircraft applications must be robust in the sense that they require high reliability and reasonably precise control, while satisfying a wide range of operating demands. The systems usually include a pump which takes fuel from a supply or sump to produce a high pressure fuel source, and various flow control components operational in the high pressure fuel circuit. Such systems are usually of fairly high mechanical complexity. A degree of leakage in the various components (between the high and low pressure sides) is virtually inherent, but within certain limits leakage can be tolerated so long as the system is able to supply adequate capacity to meet the worst case system demand. Typical prior art fuel delivery systems for use with gas turbine aircraft engine systems are shown in U.S. Pat. Nos. 3,142,154, 4,458,713 and 4,716,723, each assigned to the assignee of the present invention.
The primary purpose of the high pressure fuel delivery system is to supply high pressure fuel through a metering valve to the gas turbine engines which power the aircraft. In addition, the high pressure fuel system is often utilized as a source of high pressure fluid for the hydraulic systems which position actuators which control the engine or other aspects of the aircraft. It can thus be appreciated that the system must maintain an adequately high pressure in all circumstances, and also be capable of providing flow rates to meet the most stringent demands. Considering those requirements in the context of a relatively mechanically complex system which can tolerate a degree of internal leakage, it will be appreciated that it is difficult to predict, with any degree of certainty, the operational point at which the system as a whole will likely be incapable of meeting worst case operating demands. And since worst case operating demands are seldom encountered in actual practice, and an inadequate system will typically be capable of meeting normal operating demands, the difficulty in isolating a potentially inadequate system for repair or replacement will be apparent.
In order to adequately control fuel delivery rate to the engine by means of a metering valve, a regulating valve is provided for maintaining a substantially constant pressure across the metering valve. In normal operation such as in cruising at high altitude, the fuel demands of the engine are a relatively small fraction of the delivery capacity of the fuel system. The regulating valves in most systems serve to divert a portion of the flow from the engine metering branch in light fuel demand conditions. For example, a number of regulating valves are configured as bypass valves which simply bypass large quantities of the high pressure fuel back to the sump. While such bypassing is inefficient and tends to heat the fuel unnecessarily, it represents a practical way of dealing with the excess flow capacity in normal operating circumstances, while still assuring that capacity is present to meet stringent or worst case demands.
A particularly stringent requirement is aircraft engine starting or restarting. Even more important than starting an engine on the ground is the windmill restart of an engine on an airplane in flight. Assuming a worst case condition where the aircraft has lost power from all engines, one of the engines must be restarted while the airplane is gliding. The engine compressor and turbine blades are at that point simply "windmilling" by virtue of the air passing through the engine of the gliding aircraft. Since the fuel pump which creates the high pressure fuel supply is coupled to the engine, it will be operating at a much lower rotational speed than when the engine is normally running. Thus, the fuel system delivery capacity at starting or windmill RPM must be adequate to supply enough fuel to the engine to start it reliably. The fact that an engine has started on the ground is some indication of system operability,. but even then, it is not known how borderline the fuel delivery system might be.
When the fuel delivery system of an aircraft engine is new, its delivery capacity (and therefore its excess capacity) has been measured. As the system ages, however, the pump may become less efficient, passages may become restricted, seal leakage can increase, and in effect the maximum or average pumping capacity is not easily determined. It was pointed out above that the delivery capacity, particularly at low engine RPM, can become a critical factor. Since the aircraft operator does not know what the delivery system capacity is at any given point in its life, he is faced with two options, neither of which is completely satisfactory. The aircraft operator can simply continue to operate the equipment until it becomes difficult to start or fails to start on the ground, and use that as a measure of an obviously inadequate system. Alternatively, the delivery system can be replaced at periodic intervals somehow related to expected system wear. The former alternative can result in too great a delay before replacing the equipment, and the latter is deficient in failing to adequately take into consideration the actual condition of the equipment.